Seam Welding
Questions and Answers
The seam weld wheels perform two functions. One is to function as an electrode and deliver the force and current into the part at the desired weld interface. This is a moving location as the wheels roll.
ROLL SPOT SEAM WELDS
The second function is to move the part into position for each new weld. One or both wheels may be driven to perform this function. In some cases, the part is driven and the wheels float and follow the part movement.
In all circumstances the direction of the wheel motion at the part surface is always the same direction as the part motion.
If the part is moving into the throat of the machine. The upper and lower wheels will both turn with a motion into the throat of the machine. At the surface they are both moving in the same direction. If one wants to talk clockwise vs counterclockwise, if the upper is turning clockwise the lower is counterclockwise. If you look from the other side of the machine the opposite is true.
SEAM WELD WHEEL ROTATION
Always both wheel surface motions are in the direction of the material motion.
Reference: RWMA – Resistance Welding Manual 4th Edition
On some products it is desired to use two different sized weld wheels. They likely will be the same thickness and face contour. Due to the diameter difference the larger diameter wheel will have more material in contact
with the part surface than the smaller wheel. This reduces the current density on the large wheel surface and the heat generated there in comparison with the small wheel’s surface.
SEAM WELDER WITH TWO DIFFERENT WHEELS
The amount of this differential will depend upon the material being welded, wheel sizes and weld schedules used.
Additional discussion of this subject can be found in another article in this blog:
WHEN SEAM WELDING WITH ONE LARGE AND ONE SMALL DIAMETER WHEEL CAN THE WELD NUGGET FORM TOWARD ONE SIDE OR THE OTHER?
Reference: RWMA – Resistance Welding Manual Section 4
The lubricant used in seam welding heads is very important. It must lubricate the moving gears and parts and the conductive power components. It should be conductive and withstand heat.
KNURL DRIVEN SEAM WELDER
The specification of the lubricant depends upon the design of the product and the manufacturer. This is machine specific. This question is beyond the scope of this blog. The manufacturer of the equipment or a rebuilder should be consulted for help on this topic.
Reference: RWMA Manual Section 4
In the normal standards which I reference there is very little data for material this thin. AWS Standard C1.1 has set up data for Stainless Steel at 0.38 mm (0.015 in) thick but no testing information of the welds.
Further discussion with colleagues revealed that anything below 0.2 mm (0.008 in) is considered foil per AWS Standard D17.2 (An Aircraft Specification). Below is the table of contents for this standard AWS D17.2. It may offer some insight to this inquiry?
AWS D17.2
If the end user has not supplied a specification, then consider calling the weld either Class B or Class C and evaluate accordingly.
Reference: AWS C1.1, Recommended Practices for Resistance Welding
AWS D17.2, Specification for Resistance Welding for Aerospace Applications
There are two basic types of seam welds.
The first is a continuous seam weld.
CONTINUOUS SEAM WELD
It is a series of overlapping spot welds. They frequently would be a fluid/gas tight seal.
A roll-spot weld would look like this sample:
ROLL-SPOT SEAM WELD
The individual spot welds are spaced apart and they do not form a fluid/gas tight seal.
Let’s assume a barrel is being fabricated to hold a liquid. The steel is formed into the circular shape and the weld wheel is run down the length to form a continuous seam weld. Subsequently a lid and bottom are added and you have a barrel.
“The question is should the wheels be reversed and to reweld over the same surface a second time?”
Assuming that all was set up properly and the welds were spaced properly and the schedule produced good welds, the barrel should pass inspection after the first weld pass. The answer should be: “A SECOND PASS IS NOT NECESSARY”.
Secondly another pass over the same welds with the same settings will not weld. On the second pass the weld wheels are passing current through spot welds. The resistance is now very low and they will not heat up as they did when they were two sheets of metal. The area will get hot but not nugget forming hot.
There is a possible benefit to run a second pass. The initial pass may have created some stress or there may be a need for forging that some additional heat and pressure can improve with applied heat. Tempering or forging may be benefited by a second pass with some heat applied. The second pass may not melt nugget but it can temper or help forge the product that would benefit from same.
If you have to reverse for tempering or forging, there are machines sold on the market called a TWIN LAP WELDER that have a following seam wheel in a push/pull arrangement. The first wheel is making the seam/spot weld the second wheel follows and sees the same current but is over a solidified nugget and can only reheat the weld area for tempering or forging advantages. This eliminates the need to reverse. Both functions are performed in one pass.
If we are not doing a continuous weld but are doing a roll-spot, why reweld the same area if the first pass is set up properly and it ran well. There should be a good weld every “X” mm and the product is good to go to quality for inspection.
For additional information on seam welding see the available document at AWS listed below.
Reference: RWMA – Resistance Welding Manual 4th Edition
AWS Standard C1.1: 2019 Recommended Practices for Resistance Welding
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